Binder system method for particular material

ABSTRACT

The present invention relates to a binder composition comprising a polycarbonate polymer; an ethylenebisamide wax; and a guanidine wetting agent. The present invention further relates to a method for forming a sintered part by powder injection molding, including the steps of forming a green composition comprising a binder and an inorganic powder, wherein binder is a composition comprising a polycarbonate polymer, an ethylenebisamide wax, and a guanidine wetting agent; melting the composition; injecting the composition into a mold for a part; heating the part to a temperature at which the binder decomposes; heating the part to a temperature at which the inorganic powder is sintered. The binder composition of the present invention is useful for press and sinter applications as well as for powder injection molding applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 09/291,904, filed Apr.14, 1999, which claims the benefit of U.S. Provisional Application No.60/083,184 filed Apr. 27, 1998.

FIELD OF THE INVENTION

The present invention relates to binder compositions for use in formingsintered parts by powder injection molding and to green compositions ofthe binder composition and inorganic powders. The binders of the presentinvention require fewer steps to produce a part, have higher thixotropicenergy, melt at a lower temperature, provide a green body having highstrength, and decompose thermally in a clean, substantially ash-freeburnout to yield simple, environmentally safe products.

BACKGROUND OF THE INVENTION

Processes for forming shaped articles from particulate mixtures areknown in the art. Classically, a desired particulate material is mixedwith a binder and then formed into the desired shape, this being calleda green body. The green body is then fired to provide a fusion of theparticulate material and to drive off the binder, thereby producing thedesired shaped product with proper surface texture, strength, etc.Modern methods include press and sinter (P&S) and powder injectionmolding (PIM). In P&S, a mixture of one or more of a metal, metal oxide,intermetallic or ceramic powder and a small amount of binder (about 5%of the powder volume) are placed in a relatively simple mold, pressedinto a green body, and then sintered. The small amount of binder isdecomposed during the sintering step, so a separate step of removing thebinder is not necessary. However, P&S is limited to simple parts.

In PIM, a mixture of one or more of a metal, metal oxide, intermetallicor ceramic powder and a quantity of binder from 30% to 60% of the volumeof powder are heated to a liquid state and then injected under pressureinto a mold to form a part. Once in the mold, the binder is removed inone or more separate steps and the part is fired to sinter the particlesinto a solid part. PIM is capable of producing quite complex parts.

In the production of shaped objects by PIM in the manner abovedescribed, it has been found that the binder, while necessary to theprocess, creates problems. The binder must be used in order to form anobject of practical use, but most of it must be removed before the partcan be sintered, although in some cases a portion of the binder remainsuntil sintering is completed.

Direct removal of the PIM binder during sintering is problematic. Manybinders leave behind ash upon decomposition. When such ash combines withcertain ingredients in the powder component, eutectic mixtures may beformed. Such eutectic compounds as TiC may be formed from titanium andcarbon ash, and these can result in serious problems in the formed part.

Thermoplastic binders which decompose on heating have been used.However, these materials tend to soften or melt first and thendecompose, creating problems on decomposition. Thermoplastic materialshave been tried which decompose below their melting point and therebyremain in place until decomposition. Binders have been removed byexposure to a decomposing atmosphere, such as an acid atmosphere todecompose an acid-labile organic binder. The drawback of this approachis the use of an acid atmosphere, requiring a special chamber andhazardous material handling capabilities. Similar binders which aresubject to catalytic decomposition have been used, such as a polyacetal.The drawback of this approach is that the decomposition product isformaldehyde, which also requires special equipment to collect anddecompose the formaldehyde.

The prior art has recognized this problem and has therefore attempted toremove the binder from the shaped green body prior to the step offiring. Such processes have used various solvents, including organicsolvents, triple-point CO2, and water to dissolve and remove the binder.While systems using such procedures can provide advantages overprocedures wherein the binder is removed during firing, articles formedby removing the binder prior to firing still have the tendency to crackduring the binder removal as well as during the firing operation. Onereason for this is that the binder is removed from the green body bymeans of a solvent when the binder is In the solid state, and upondissolution the binder, the binder-solvent mixture has a tendency toexpand. This problem has been approached by various means, includingheating the green body prior to exposing it to the solvent, by using asolvent to remove a portion of the binder and removing the remainder byfiring, and by using a two-part binder, each part of which is soluble ina different solvent, so each solvent removes a portion of the binder,and by using the different solvents in a stepwise manner, Each of thesemethods includes its own drawbacks.

Thus, the need remains for binders which are useful, particularly inpowder injection molding, which require a minimum number of steps toremove, which have high thixotropic energy, which melt at a lowtemperatures, which provide a green body having high strength, and whichdecompose thermally to yield simple, environmentally safe products,substantially free of ash, thereby yielding a binder which performs itsfunction but which provides a process of powder injection molding whichproceeds with a minimum number of process steps, can be carried out inan air atmosphere in many cases, and does not leave behind deleteriousresidues, either in the part or in the environment. The presentinvention requires only simple, standard equipment which is inexpensiveand commonly available. The steps of debinding and sintering may becarried out in the same equipment, on a continuous basis, therebyavoiding downtime for cooling and transfer from debinding equipment tosintering equipment.

SUMMARY OF THE INVENTION

The present invention relates to a binder composition comprising apolycarbonate polymer; an ethylenebisamide wax; and a guanidine wettingagent. The present invention further relates to a method for forming asintered part by powder Injection molding, including the steps offorming a green composition comprising a binder and an inorganic powder,wherein binder is a composition comprising a polycarbonate polymer, anethylenebisamide wax, and a guanidine wetting agent; transferring thegreen composition into a mold for a part; heating the part to atemperature at which the binder decomposes; and heating the part to atemperature at which the inorganic powder is sintered.

Thus, the binder composition and method of making sintered parts usingthe binder composition of the present invention provide the featuresmissing from the prior art. The binder composition may be removed in aminimum number of steps, has high thixotropic energy, melts and becomesflowable at a low temperature, provides a green body having highstrength, and decomposes thermally to yield simple, environmentally safeproducts, substantially free of ash. The binder composition therebyperforms its function while providing a process of powder injectionmolding which proceeds with a minimum number of steps, can be debound inair, hydrogen, oxygen, argon, nitrogen and similar gas atmospheres or invacuum, and does not leave behind deleterious residues, either in thepart or in the environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the steps in a method of making a partby powder injection molding in accordance with the present invention.

FIG. 2 is a graph of a debinding profile of a first exemplary greencomposition according to the present invention.

FIG. 3 is a graph of a debinding profile of a second exemplary greencomposition according to the present invention.

FIG. 4 is a graph of a debinding and partial sintering profile of athird exemplary green composition according to the present invention.

FIG. 5 is a schematic engineering drawing of one screw of a twin screwcompounding extruder in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

The binder composition and the green composition comprising the bindercomposition and an inorganic powder, each in accordance with the presentinvention, are applicable both to powder injection molding (PIM)techniques and to press and sinter (P&S) applications. In PIM, a greencomposition or feedstock comprising an inorganic powder and a bindercomposition is used for powder injection molding, which includes stepsof debinding and sintering. In P&S applications, a green compositioncomprising an inorganic powder and a binder composition are pressed intoa mold and sintered to form a part, without a step of debinding. Theinorganic powders which may be used in the green compositions and methodof the present invention may be metal, metal oxide, intermetallic and/orceramic, or mixtures of these, depending upon the desiredcharacteristics of the final product. The green composition comprisingan inorganic powder and binder composition of the present invention, maybe injection molded with an increased loading of the powder compared toprior processes, resulting in less shrinkage and deformation duringdebinding and sintering. The components of the binder composition allowdebinding of the nascent part with decomposition of the binder to yieldenvironmentally safe products in a relatively rapid, controllableprocess, thereby efficiently overcoming the deficiencies of the priorart.

The components of the binder composition are partially miscible with oneanother, such that when the green composition is ready for use, thecomponents thereof are sufficiently miscible that the desired parts areformed when the composition is pumped into the mold, but the componentsare sufficiently immiscible that the phases can separate and thecomponents will “come apart” in a step-wise, controllable manner in anoven or kiln during the debinding step. The binder composition of thepresent invention may be removed thermally, in the same oven or chamberin which the part is sintered, thereby avoiding a multiple oven,multiple step process of debinding and sintering the part.

The present inventor has discovered that the components of the bindercomposition controllably debind in an order which is the opposite ofthat normally sought in the PIM industry. In conventional bindercompositions, which include, e.g., stearic acid as a surface agent,paraffin wax as the wax, and polypropylene as the major bindercomponent, during the debinding step of a PIM process, the surface agentreleases first, the wax component releases next, and the major bindercomponent releases last.

The components of the binder composition of the present invention, incontrast, release in the opposite order. In the binder composition ofthe present invention, the major binder component, a polycarbonatepolymer, has a decomposition temperature of about 185° C. The waxcomponent, an ethylenebisamide wax, has a decomposition temperature ofabout 285° C. The guanidine wetting or surface agent is the lastcomponent to decompose, having a decomposition temperature in the rangeof about 350° C. to about 450° C. Thus, according to the presentinvention, during the debinding step of a PIM process, the components ofthe binder composition debind in an order opposite to that ofconventional binder compositions.

As a result of the debinding profile of the binder composition accordingto the present invention, the surface agent, is the last to decompose inthe debinding step. As a result, the inorganic powder is retained inposition for a longer time in the pre-sintering portion of the process.Retaining the inorganic powder particles in position for a longer timeprovides the benefit of allowing the transition from debinding tosintering to occur with a significantly reduced possibility that theinorganic powder particles will move or be distorted from their originalposition in the mold. As a result, superior sintered parts are obtainedfrom the PIM process using the binder composition of the presentinvention.

The partial miscibility of the components of the binder compositionfacilitates the reverse debinding of the present invention. Since thepolycarbonate polymer is only partially miscible with the othercomponents and has a lower glass transition (T_(g)) and melting ordecomposition temperature, it can melt and separate from the othercomponents of the binder composition, then wick out of the green partfirst. When the polycarbonate component has been removed, thetemperature may be raised to a temperature at which the next componentmay be debound. In the present invention, the ethylenebisamide is thenext component to decompose or be debound. Again the partial miscibilityof the components aids the separation, allowing the ethylenebisamide todecompose with affecting the guanidine wetting agent. When theethylenebisamide has been removed, only the guanidine wetting agentremains. At this time, the temperature is again increased to thedecomposition temperature of the guanidine wetting agent, which is inthe range from about 350° C. to about 450° C., depending on the exactnature of the guanidine wetting agent, i.e., which acid has been reactedwith guanidine to form the guanidine wetting agent, Once the guanidinewetting agent has been debound, the remaining Inorganic powder may besintered to form the desired final part.

The binder composition of the present invention comprises apolycarbonate polymer, a wax such as ethylenebisamide wax, and aguanidine wetting agent. Each of these three general component materialsis more fully disclosed in the following.

GUANIDINE WETTING AGENT

In one embodiment, the guanidine wetting agent is a reaction product ofguanidine and an acid selected from a fatty acid, an organic acid, acidand a stronger acid such as an alkyl sulfonic acid. The guanidinewetting is a reaction product which may be an amide or actually may bemore in the nature of a hydrated salt. For example, according to the CRCHandbook of Chemistry and Physics, 74^(th) Ed., guanidine acetate hasthe formula (H₂N)₂C═NH•CH₃COOH, rather than an amide-type formula suchas H₂N—C═NH(NH)COCH₃, as would be expected for an amide. This is due tothe fact that guanidine is a very strong base, and is much more likelyto simply abstract a proton from a relatively weak organic acid, ratherthan react with the organic acid in a “standard” amidization reactionforming an amide with concomitant loss of H₂O. However, in some cases,the reaction of guanidine and the acid may yield an amide in the“standard” manner. For this reason, the guanidine surface agent of thepresent invention will be referred to herein as the reaction product ofguanidine and an acid. The term “reaction product of guanidine and anacid” includes both of the above-described forms of the product of areaction between or mixture of guanidine and an acid, and mixtures ofthese forms or other possible forms.

The particular acid used to make the reaction product of guanidine andan acid is selected based upon the surface charge of the inorganicpowder with which the binder composition is to be used. In oneembodiment, the guanidine wetting agent is guanidine stearate. Guanidinestearate and guanidine compounds of similar acids are selected for usewith powders having a positive surface charge and an isoelectric pointat a low pH. In one embodiment, the guanidine wetting agent is guanidineethyl-hexanoate. Guanidine ethyl-hexanoate and guanidine compounds ofsimilar acids are selected for use with powders having an amphotericsurface charge, and an isoelectric point at a near-neutral pH. In oneembodiment, the guanidine wetting agent is guanidine lauryl sulfonate.Guanidine lauryl sulfonate and guanidine compounds of similar acids areselected for use with powders having a negative surface charge, and anisoelectric point at a high pH. In other embodiments, the guanidinewetting agent may be the reaction product of guanidine and other acids.The selection of the appropriate acid for preparation of the reactionproduct of guanidine and an acid depends upon the isoelectric point ofthe inorganic powder. The relationship is further described in thefollowing detailed description. The many acids which may be reacted withthe guanidine to form the reaction product of guanidine and an acid aredescribed in detail hereafter.

In general, the appropriate acid depends on the surface charge, or pointof zero charge (PZC), which may be expressed as the isoelectric point(IEP) of the inorganic powder with which the binder composition is to beused in the green composition. Isoelectric points may be found inreference sources, or may be determined experimentally, by determiningthe pH at which no charge exists on the powder particle. The point ofzero charge is the average of the pK's for the particular powder, andindicates the average acid-base character of the surface. Isoelectricpoints of a number of ceramic oxide materials are shown in the followingtable:

TABLE Isoelectric Points of Oxides Nominal Material Composition IEPMuscovite KAl₃Si₃O₁₁H₂O₁₁ 1 Quartz SiO₂ 2 Delta manganese oxide MnO₂ 2Soda lime silica glass 1.00 Na₂O 2-3 0.58 CaO 3.70 SiO₂ Aibite Na₂OAl₂O₃ 6 SiO₂ 2 Orthoclase K₂O A1₂O₃6 SiO₂ 3-5 Silica (amorphous) SiO₂3-4 Zirconia ZrO₂ 4-5 Rutile TiO₂ 4-5 Tin Oxide SnO₂ 4-7 Apatite 10 CaO6 PO₂ 4-6 2 H₂O Zircon SiO₂ ZrO₂ 5-6 Anatase TiO₂ 6 Magnetite Fe₃O₄ 6-7Hematite αF₃O₃ 6-9 Goethite FeOOH 6-7 Gamma iron oxide γFe₂O₃ 6-7 kaolin(edges) Al₂O₃ SiO₂ 2 H₂O 6-7 Chromium oxide αCr₂O₃ 6-7 Mullite 3 Al₂O₃ 2SiO₂ 7-8 Gamma alumina γAl₂O₃ 7-9 Alpha alumina αAl₂O₃   9-9.5 Alumina(Bayer process) Al₂O₃   7-9.5 Zinc oxide ZnO₂ 9 Copper oxide CuO 9Barium carbonate BaCO₃ 10-11 Yttria Y₂O₃ 11  Lathanum oxide La₂O₃ 10-12Silver oxide Ag₂O 11-12 Magnesium Oxide MgO 12-13 Source: Temple C.Patton, Paint Flow and Pigment Dispersion, Wiley-Interscience. New York,1979; E. G. Kelly and D. J. Spottiswood, Introduction to MineralProcessing, Wiley-Interscience, New York, 1982; 1. M. Cases, Silic.Ind., 36, 145 (1971); R. H. Toon, T. Salman, and G. Donnay, J. ColloidInterface Sci., 70, 483 (1979).

According to the present invention, the reaction product of guanidineand organic acids in the C₁₂ to C₂₂ range are used with materials havinga low isolectric point, i.e., which have a low pH at the point of zerocharge. Thus, for example the reaction product of guanidine and oleicacid (C₁₇H₃₃CO₂H) would be suitable for use with quartz powder (SiO₂),which has an IEP of 2, according to Table 1 above. Other suitable acidsfor use with inorganic powders having a low isoelectric point includesuch saturated fatty acids as (common names in parentheses) dodecanoic(lauric) acid, tridecanoic (tridecylic) acid, tetradecanoic (myristic)acid, pentadecanoic (pentadecylic) acid, hexadecanoic (palmitic) acid,heptadecanoic (margaric) acid, octadecanoic (stearic) acid, eicosanoic(arachidic) acid, 3,7,11,15-tetramethylhexadecanoic (phytanic) acid,monounsaturated, diunsaturated, triunsaturated and tetraunsaturatedanalogs of the foregoing saturated fatty acids.

According to the present invention, the reaction product of guanidineand organic acids in the C₆ to C₁₂ range are used with materials havinga mid-range isolectric point, i.e., which have a pH around 6 at thepoint of zero charge.

These materials may also be referred to as amphoteric. For example, thereaction product of guanidine and an organic acid such as ethylhexanoicacid (C₇H₁₅CO₂H) would be suitable for use with an inorganic powderhaving an IEP of about 6.0, for example with zircon (SiO₂•ZrO₂), whichhas an IEP of 5-6, or anatase (TiO₂), which has an IEP of 6, eachaccording to Table 1 above. Hexanoic acid, heptanoic acid, octanoicacid, nonanoic acid, decanoic acid, dodecanoic acid are otherstraight-chain carboxylic acids which may be reacted with guanidine foruse with inorganic powders having a mid-range isoelectric point.Branched-chain carboxylic acids in the C₆ to C₁₂ range may also be usedwith materials having a mid-range isolectric point.

According to the present invention, the reaction product of guanidineand stronger acids such as sulfonates, phthalates, benzoates, phosphatesand phenols are used with materials having a high isolectric point,i.e., which have a pH around 10-12 at the point of zero charge. Forexample, the reaction product of guanidine and an acid such asbenzenesulfonic acid may be used with an inorganic powder such as silveroxide, which has an IEP of 11-12, as shown in Table 1 above.

According to the present invention, for materials having intermediateIEPs, such as, for example, mullite (3Al₂O₃•2SiO₂), IEP=7-8, a mixtureof guanidine wetting agents may be used. As an alternative, intermediateacids may be selected for reaction with guanidine. Thus, for example, ifmullite is the inorganic powder, the guanidine wetting agent used in thebinder composition therewith may be a mixture of the reaction product ofguanidine and benzenesulfonic acid and the reaction product of guanidineand ethylhexanoic acid. Alternatively, for mullite, the guanidinewetting agent used in the binder composition therewith may be thereaction product of guanidine and a weaker acid such as benzoic acid maybe used.

Similarly, according to the present invention, for materials havingintermediate IEPs, such as, for example, amorphous silica (SiO₂),IEP=3-4, a mixture of guanidine wetting agents may be used. As analternative, intermediate acids may be selected for reaction withguanidine. Thus, for example, if silica is the Inorganic powder, theguanidine wetting agent used in the binder composition therewith may bea mixture of the reaction product of guanidine and octadecanoic(stearic) acid and the reaction product of guanidine and ethylhexanoicacid. Alternatively, for silica, the guanidine wetting agent used in thebinder composition therewith may be the reaction product of guanidineand dodecanoic acid may be used. Dodecanoic acid, C₁₁H₂₃CO₂H, appears inboth groups of acids, those for use with the low IEP powders and thosefor use with intermediate IEP powders. The intermediate character ofsuch an acid makes it suitable for use with an intermediate IEP powder.

While a certain amount of trial and error may be required to optimizethe reaction product of guanidine and an acid for a particular inorganicpowder, and particulary for a combination of inorganic powders, theselection can be guided by the foregoing disclosure. Thus, the low IEPpowders work best with a “very fatty”, relatively weak acid,intermediate IEP powders work best with a mid-range organic acid, andthe high EIP powders work best with a stronger acid having relativelyless organic character, such as an alkyl sulfonic acid. The acidselected should be rheologically compatible with the compounding andinjection molding equipment. Some testing may be required in order tooptimize the acid for reaction with guanidine to form the guanidinewetting agent for a given inorganic powder.

POLYCARBONATE POLYMER

In the binder composition of the present invention, the polycarbonatepolymer is a low molecular weight polycarbonate polymer. In oneembodiment, the polycarbonate polymer Is poly(propylene carbonate).Poly(propylene carbonate) is prepared from the reaction of carbondioxide, CO2, and propylene oxide, CH₂═C(O)CH₂, as shown in thefollowing:

The poly (propylene carbonate) shown above, on application of sufficientheat, decomposes cleanly into the following, which is a liquid having aboiling point near the decomposition temperature of the poly (propylenecarbonate):

In one embodiment, the polycarbonate polymer has a number averagemolecular weight in the range from about 25,000 to about 75,000. In oneembodiment, the polycarbonate polymer has a number average molecularweight in the range from about 35,000 to about 65,000. In oneembodiment, the polycarbonate polymer has a number average molecularweight in the range from about 35,000 to about 40,000. In oneembodiment, the polycarbonate polymer has a number average molecularweight of about 50,000. In one embodiment, the polycarbonate polymer hasa number average molecular weight in the range from about 45,000 toabout 55,000.

In one embodiment, the polycarbonate polymer is Q-PAC™ 40, availablefrom PAC Polymers, a division of Axcess Corporation, Newark, DE. Q-PAC™40 is a low molecular weight polycarbonate, having a number averagemolecular weight in the range of about 50,000. Q-PAC™ 40 has a glasstransition temperature, T_(g)=40° C. Q-PAC™ 40 is a low boiling liquid,having a boiling point of 242° C. Thus, at relatively moderatetemperatures, Q-PAC™ 40 melts and may exit the green form as a liquidhaving only a slightly increased volume with respect to the solid,rather than decomposing into a gas having a greatly increased volumewith respect to the solid. As above, the partial miscibility of thepolycarbonate polymer allows it to melt and separate from the remainingcomponents of the green composition during the debinding process.

The decomposition product of poly (propylene carbonate) is shown above.This cyclic propylene carbonate has a melting point below thetemperature at which the polymer decomposes. Thus, as the bindercomposition of the present invention, when mixed with the inorganicpowder to form the green composition and injected into a mold, isheated, the poly (propylene carbonate) first melts and then begins todecompose into the liquid cyclic propylene carbonate shown above. Onfurther heating in the debinding process, the cyclic propylene carbonatedecomposes cleanly in air to form CO₂ and water. Thus, according to thepresent invention, the polycarbonate polymer is the first component tobe lost from the green composition in the debinding process. Incontrast, in the prior art binders, the polymeric component has beendesigned to be the last component lost from the binder during thedebinding process.

ETHYLENEBISAMIDE WAX

The binder composition of the present invention includes anethylenebisamide wax. The ethylenebisamide wax is a wax formed by theamidization reaction of ethylene diamine and a fatty acid. The fattyacid may be in the range from C₁₂ to C₂₂, but is usually made fromstearic acid, the saturated C₁₈ fatty acid. Thus, in one embodiment, theethylenebisamide wax is ethylenebisstearamide wax. Ethylenebisstearamidehas a discrete melting point of about 142° C. In one embodiment, theethylenebisamide wax has a discrete melting point in the range fromabout 120° C. to about 160° C. In one embodiment, the ethylenebisamidewax has a discrete melting point in the range from about 130° C. toabout 150° C. In one embodiment, the ethylenebisamide wax has a discretemelting point of about 140° C.

In one embodiment, the ethylenebisstearamide is ACRAWAX® C, availablefrom LONZA, Inc. ACRAWAX® C has a discrete melt temperature of 142° C.

In other embodiments of the binder composition, other ethylenebisamidesinclude the bisamides formed from the fatty acids ranging from C₁₂ toC₃₀. Illustrative of these acids are lauric acid, palmitic acid, oleicacid, linoleic acid, linolenic acid, oleostearic acid, stearic acid,myristic acid, and undecalinic acid. Unsaturated forms of these fattyacids may also be used.

QUANTITIES OF COMPONENTS IN THE BINDER AND GREEN COMPOSITIONS

It is a practice in the art of powder metal to refer to a bindercomposition in terms of parts by weight, or percent of each component ona weight basis, and to refer to a green composition in terms of parts byvolume, or percent of each component on a volume bases. Thus, the amountof each component in the binder composition is expressed as weightpercent, or wt %. The amounts of the inorganic powder and the bindercomposition combined to form the green composition are expressed asvolume percent, or vol %. This practice is followed throughout thepresent specification and claims.

In one embodiment, the binder composition comprises the guanidinewetting agent in the range from about 5 wt % to about 30 wt % based onthe binder composition, the polycarbonate polymer in the range fromabout 30 wt % to about 85 wt % based on the binder composition, and theethylenebisamide wax in the range from about 10 wt % to about 40 wt %based on the binder composition. In one embodiment of the bindercomposition, the guanidine wetting agent is present at about 15.5 wt %,the polycarbonate polymer is present at about 59.4 wt %, andethylenebisstearamide is present at about 25.1 wt %, each weight percentbased on the binder composition. In one embodiment, the polycarbonatepolymer is Q-PAC™ 40 brand of poly(propylene carbonate), and is presentat about 60 wt %. In one embodiment, the ethylenebisamide is ACRAWAX® Cbrand of ethylenebisstearamide, and is present at about 25 wt %.

In one embodiment, the binder composition comprises the guanidinewetting agent in the range from about 10 wt % to about 25 wt % based onthe binder composition, the polycarbonate polymer in the range fromabout 40 wt % to about 60 wt % based on the binder composition, and theethylenebisamide wax in the range from about 15 wt % to about 35 wt %based on the binder composition.

The binder composition of the present invention may also be used for P&Sapplications. In such applications, the binder composition comprises theguanidine wetting agent in the range from about 5 wt % to about 30 wt %based on the binder composition, the polycarbonate polymer in the rangefrom about 10 wt % to about 50 wt % based on the binder composition, andthe ethylenebisamide wax in the range from about 30 wt % to about 70 wt% based on the binder composition.

The binder composition of the present invention is designed to becombined with an inorganic powder, to form a green composition for usein PIM. In one embodiment, the green composition includes the bindercomposition, as described above, and at least one inorganic powderselected from a metal powder, a metal oxide powder, a non-metallicpowder and a ceramic powder. In one embodiment, the green compositionincludes the binder composition in an amount in the range from about 30vol % to about 60 vol % and the inorganic powder or powders in an amountfrom about 70 vol % to about 40 vol %. In one embodiment, the greencomposition includes the binder composition in an amount in the rangefrom about 40 vol % to about 50 vol % and the inorganic powder ispresent in an amount from about 60 vol % to about 50 vol %. In oneembodiment, the green composition includes the binder composition in anamount of about 35 vol % and the inorganic powder in an amount of about65 vol %.

The binder composition of the present invention is also suitable for usewith an inorganic powder, to form a green composition for use in P&S. Inone embodiment, the green composition includes the binder composition,as described above, and at least one inorganic powder selected from ametal powder, a metal oxide powder, a non-metallic powder and a ceramicpowder. In one embodiment, the green composition includes the bindercomposition in an amount in the range from about 1 vol % to about 10 vol% and the inorganic powder or powders in an amount from about 99 vol %to about 90 vol %. In one embodiment, the green composition includes thebinder composition in an amount in the range from about 2 vol % to about5 vol % and the inorganic powder is present in an amount from about 98vol % to about 95 vol %. In one embodiment, the green compositionincludes the binder composition in an amount of about 2.5 vol % and theinorganic powder in an amount of about 97.5 vol %.

INORGANIC POWDERS

Inorganic powders used in the present invention include metallic, metaloxide, intermetallic and/or ceramic powders. The powders may be oxidesor chalcogenides of metallic or non-metallic elements. An example ofmetallic elements which may be present in the inorganic powders includecalcium, magnesium, barium, scandium, titanium, vanadium, chromium,manganese, iron, cobalt, nickel, copper, zinc, yttrium, niobium,molybdenum, ruthenium, rhodium, silver, cadmium, lanthanum, actinium,gold or combinations of two or more thereof. In one embodiment, theinorganic powder may contain rare earth or ferromagnetic elements. Therare earth elements include the lanthanide elements having atomicnumbers from 57 to 71, inclusive and the element yttrium, atomic number39.

Ferromagnetic metals, for purposes of this invention, include iron,nickel, cobalt and numerous alloys containing one or more of thesemetals. In another embodiment, the metals are present as alloys of twoor more of the aforementioned elements. In particular, prealloyedpowders such as low alloy steel, bronze, brass and stainless steel aswell as nickel-cobalt based super alloys may be used as inorganicpowders.

The inorganic powders may comprise inorganic compounds of one or more ofthe above-described metals. The inorganic compounds include ferrites,titanates, nitrides, carbides, borides, fluorides, sulfides, hydroxidesand oxides of the above elements. Specific examples of the oxide powdersinclude, in addition to the oxides of the above-identified metals,compounds such as beryllium oxide, magnesium oxide, calcium oxide,strontium oxide, barium oxide, lanthanum oxide, gallium oxide, indiumoxide, selenium oxide, zinc oxide, aluminum oxide, silica, zirconia,mullite, mica, indium tin oxide, rare earth oxides, titania, yttria,etc. Specific examples of oxides containing more than one metal,generally called double oxides, include perovskite-type oxides such asNaNbO₃, SrZrO₃, PbZrO₃, SrTiO₃, BaZrO₃, BaTiO₃; spinel-type oxides suchas MgAl₂O₄, ZnAl₂O₄, CoAl₂O₄, NiAl₂O₄, NiCr₂O₄, FeCr₂O₄, MgFe₂O₄,ZnFe₂O₄, etc.; illmenite-types oxides such as MgTiO₃ MnTiO₃, FeTiO₃,CoTiO₃, ZnTiO₃, LiTaO₃, etc.; and garnet-type oxides such as Gd₃Ga₅O₁₂and rare earth-iron garnet represented by Y₃Fe₅O₁₂. The inorganic powdermay also be a clay. Examples of clays include kaolinite, nacrite,dickite, montmorillonite, montronite, spaponite, hectorite, etc.

An example of non-oxide powders include carbides, nitrides, borides andsulfides of the metals described above. Specific examples of thecarbides include SiC, TiC, WC, TaC, HfC, ZrC, AlC; examples of nitridesinclude Si₃N₄, AlN, BN and Ti₃N₄; and borides include TiB₂, ZrB₂, B₄Cand LaB₆. In one embodiment, the inorganic powder is silicon nitride,silicon carbide, zirconia, alumina, aluminum nitride, barium ferrite,barium-strontium ferrite or copper oxide. In another embodiment, thepowder is a semiconductor, for example, GaAs, Si, Ge, Sn, AlAs, AlSb,GaP, GaSb, InP, InAs, InSb, CdTe, HgTe, PbSe, PbTe, and any of the manyother known semiconductors. In another embodiment, the Inorganic powderis alumina or clay.

ACIDS FOR REACTION WITH GUANIDINE

The acidic compounds useful in making the reaction product of guanidineand an acid of the present invention include carboxylic acids, sulfonicacids, phosphorus acids, phenols or mixtures of two or more thereof.Preferably, the acidic organic compounds are carboxylic acids orsulfonic acids. The carboxylic and sulfonic acids may have substituentgroups derived from the above described polyalkenes. Selection criteriafor the appropriate acid are provided above, based on the surface chargeand isoelectric point of the inorganic powder used in preparing thegreen composition.

The carboxylic acids may be aliphatic or aromatic, mono- orpolycarboxylic acid or acid-producing compounds. The acid-producingcompounds include anhydrides, lower alkyl esters, acyl halides, lactonesand mixtures thereof unless otherwise specifically stated.

Illustrative fatty carboxylic acids include palmitic acid, stearic acid,myristic acid, oleic acid, linoleic acid, behenic acid,hexatriacontanoic acid, tetrapropylenyl-substituted glutaric acid,polybutenyl (Mn=200-1,500, preferably 300-1,000)-substituted succinicacid, polypropylenyl, (Mn=200-1,000, preferably 300-900)-substitutedsuccinic acid, octadecyl-substituted adipic acid, 9-methylstearic acid,stearyl-benzoic acid, eicosane-substituted naphthoic acid,dilauryl-decahydronaphthalene carboxylic acid, mixtures of these acids,and/or their anhydrides. Aliphatic fatty acids include the saturated andunsaturated higher fatty acids containing from about 12 to about 30carbon atoms. Illustrative of these acids are lauric acid, palmiticacid, oleic acid, linoleic acid, linolenic acid, oleostearic acid,stearic acid, myristic acid, and undecalinic acid, alpha-chlorostearicacid, and alphanitrolauric acid. Branched fatty acids, both saturatedand unsaturated, in the range from about 6 to about 25 carbon atoms areincluded. Such branched fatty acids include versatic acids, availablefrom Shell Chemicals. For example, Shell Chemical produces a versaticacid known as Monomer Acid, which is the distilled product obtainedduring the manufacture of tall oil-based dimer acid. Monomer Acid is amixture of both branched and straight-chain predominantly C₁₈ mono fattyacids. One example is Versatic 10, a synthetic saturated monocarboxylicacid of highly branched structure containing ten carbon atoms. Itsstructure may be represented as:

where R1, R2 and R3 are alkyl groups at least one of which is alwaysmethyl.

The sulfonic acids useful in making the guanidine wetting agents includethe sulfonic and thiosulfonic acids. Generally they are salts ofsulfonic acids. The sulfonic acids include the mono- or polynucleararomatic or cycloaliphatic compounds. The oil-soluble sulfonates can berepresented for the most part by one of the following formulae:R⁷—T—(SO₃)_(d) and R⁸—(SO₃)_(e), wherein T is a cyclic nucleus such as,for example, benzene, naphthalene, anthracene, diphenylene oxide,diphenylene sulfide, petroleum naphthenes, etc.; R⁷ is an aliphaticgroup such as alkyl, alkenyl, alkoxy, alkoxyalkyl, etc.; (R⁷)+T containsa total of at least about 15 carbon atoms; R⁸ is an aliphatichydrocarbyl group containing at least about 15 carbon atoms and d and eare each independently an integer from 1 to about 3, preferably 1.Examples of R⁸ are alkyl, alkenyl, alkoxyalkyl, carboalkoxyalkyl, etc.Specific examples of R⁸ are groups derived from petrolatum, saturatedand unsaturated paraffin wax, and the above-described polyalkenes. Thegroups T, R⁷, and R⁸ in the above formulae can also contain otherinorganic or organic substituents in addition to those enumerated abovesuch as, for example, hydroxy, mercapto, halogen, nitro, amino, nitroso,sulfide, disulfide, etc. In the above Formulae, d and e are at least 1.

Illustrative examples of these sulfonic acids includemonoeicosane-substituted naphthalene sulfonic acids, dodecylbenzenesulfonic acids, didodecylbenzene sulfonic acids, dinonylbenzene sulfonicacids, cetylchlorobenzene sulfonic acids, dilauryl beta-naphthalenesulfonic acids, the sulfonic acid derived by the treatment ofpolybutenyl, having a number average molecular weight (Mn) in the rangeof about 500, preferably about 800 to about 5000, preferably about 2000,more preferably about 1500, with chlorosulfonic acid, nitronaphthalenesulfonic acid, paraffin wax sulfonic acid, cetyl-cyclopentane, sulfonicacid, lauryl-cyclohexane sulfonic acids, polyethylenyl (Mn=300-1,000,preferably 750) sulfonic acids, etc. Normally the aliphatic groups willbe alkyl and/or alkenyl groups such that the total number of aliphaticcarbons is at least about 8, preferably at least 12.

A preferred group of sulfonic acids are mono-, di-, and tri-alkylatedbenzene and naphthalene (including hydrogenated forms thereof) sulfonicacids. Illustrative of synthetically produced alkylated benzene andnaphthalene sulfonic acids are those containing alkyl substituentshaving from about 8 to about 30 carbon atoms, preferably about 12 toabout 30 carbon atoms, and advantageously about 24 carbon atoms. Suchacids include di-isododecyl-benzene sulfonic acid,polybutenyl-substituted sulfonic acid, polypropylenyl-substitutedsulfonic acids of Mn=300-1000, preferably 500-700, cetylchlorobenzenesulfonic acid, di-cetyinaphthalene sulfonic acid, di-lauryidiphenylethersulfonic acid, diisononylbenzene sulfonic acid, di-isooctadecylbenzenesulfonic acid, stearyinaphthalene sulfonic acid, and the like.

The production of sulfonates from detergent manufactured by-products byreaction with, e.g., SO₃, is well known to those skilled in the art.See, for example, the article “Sulfonates” in Kirk-Othmer “Encyclopediaof Chemical Technology”, Second Edition, Vol. 19, pp. 291 et seq.published by John Wiley & Sons, New York (1969).

The phosphorus-containing acids useful in making the guanidine wettingagents include any phosphorus acids such as phosphoric acid or esters;and thiophosphorus acids or esters, including mono and dithiophosphorusacids or esters. Preferably, the phosphorus acids or esters contain atleast one, preferably two, hydrocarbyl groups containing from 1 to about50 carbon atoms, typically 1, preferably 3, more preferably about 4 toabout 30, preferably to about 18, more preferably to about 8.

In one embodiment, the phosphorus-containing acids are dithiophosphoricacids which are readily obtainable by the reaction of phosphoruspentasulfide (P₂S₅) and an alcohol or a phenol. The reaction involvesmixing at a temperature of about 20° C. to about 200° C. four moles ofalcohol or a phenol with one mole of phosphorus pentasulfide. Hydrogensulfide is liberated in this reaction. The oxygen-containing analogs ofthese acids are conveniently prepared by treating the dithioic acid withwater or steam which, in effect, replaces one or both of the sulfuratoms with oxygen.

In one embodiment, the phosphorus-containing acid is the reactionproduct of the above polyalkenes and phosphorus sulfide. Usefulphosphorus sulfide-containing sources include phosphorus pentasulfide,phosphorus sesquisulfide, phosphorus heptasulfide and the like.

The reaction of the polyalkene and the phosphorus sulfide generally mayoccur by simply mixing the two at a temperature above 80° C., preferablybetween 100° C. and 300° C. Generally, the products have a phosphoruscontent from about 0.05% to about 10%, preferably from about 0.1% toabout 5%. The relative proportions of the phosphorus sulfide to theolefin polymer is generally from 0.1 part to 50 parts of the phosphorussulfide per 100 parts of the olefin polymer.

The phenols useful in making the guanidine wetting agents may berepresented by the formula (R)f —Ar—(OH)g, wherein R and Ar are definedabove; f and g are independently numbers of at least one, the sum of fand g being in the range of two up to the number of displaceablehydrogens on the aromatic nucleus or nuclei of Ar. Preferably, f and gare independently numbers in the range of 1 to about 4, more preferably1 to about 2. R and f are preferably such that there is an average of atleast about 8 aliphatic carbon atoms provided by the R groups for eachphenol compound. Examples of phenols include octylphenol, nonylphenol,propylene tetramer substituted phenol, tri(butene)-substituted phenol,polybutenyl-substituted phenol and polypropenyl-substituted phenol.

OTHER ADDITIVES

Other additives used in prior art binder compositions are not necessarywith the binder composition of the present invention. In one embodiment,no additives beyond the inventive binder composition are used. In oneembodiment, as deemed necessary, small amounts of other materials may beadded to the composition of the present invention. For example,plasticizers may be added to the compositions to provide more workablecompositions. Examples of plasticizers normally utilized in inorganicformulations include dioctyl phthalate, dibutyl phthalate, benzyl butylphthalate and phosphate esters.

METHODS

The present invention further relates to a method for forming a part bypowder injection molding, comprising the steps of (a) forming a greencomposition comprising a binder composition and an inorganic powder,wherein the binder composition comprises a polycarbonate polymer, anethylenebisamide wax, and a guanidine wetting agent, (b) transferringthe green composition into a mold for a part, (c) heating the part to atemperature at which the binder composition decomposes, (d) heating thepart to a temperature at which the powder is sintered to form the part,and (e) cooling and removing the part from the mold. In one embodiment,the transferring step (b) includes heating and injection of the greencomposition into a mold for powder injection molding. In one embodiment,the transferring step (b) includes gravity feeding the green compositioninto a mold for press & sinter molding. In one embodiment of the method,the heating step (d) is performed as a series of temperature increasesto selected temperatures, in which the selected temperatures correspondto debinding temperatures of the components in the binder composition.In one embodiment, the selected temperatures are held for a period oftime, to allow the component to be debound prior to increasing thetemperature to a debinding temperature of another component. In oneembodiment of the method, the order of debinding is polycarbonatepolymer first, ethylenebisamide second, and guanidine wetting agentlast. In one embodiment, a wicking agent may be used in the debindingstep. In one embodiment, the wicking agent may be used in both thedebinding step and the sintering step. The wicking agent may be, forexample, a fine alumina or zirconia sand.

In one embodiment of the method, the inorganic powder is selected from ametal powder, a metal oxide powder, a non-metallic powder and a ceramicpowder. In one embodiment of the method, the guanidine wetting agent isa reaction product of guanidine and an acid selected from organic acid,a fatty acid and a stronger acid such as an alkyl sulfonic acid. In oneembodiment of the method, the guanidine wetting agent is guanidinestearate. In one embodiment of the method, the guanidine wetting agentis guanidine ethyl-hexanoate. In one embodiment of the method, theguanidine wetting agent is guanidine lauryl sulfonate.

In one embodiment of the method, the polycarbonate polymer has a numberaverage molecular weight in the range from about 25,000 to about 50,000.In one embodiment of the method, the polycarbonate polymer has a numberaverage molecular weight in the range from about 30,000 to about 45,000.In one embodiment of the method, the polycarbonate polymer has a numberaverage molecular weight in the range from about 35,000 to about 40,000.

In one embodiment of the method, the ethylenebisamide wax has a discretemelting point in the range from about 120° C. to about 160° C. In oneembodiment of the method, the ethylenebisamide wax has a discretemelting point in the range from about 130° C. to about 150° C. In oneembodiment of the method, the ethylenebisamide wax has a discretemelting point of about 140° C. In one embodiment of the method, theethylenebisamide is ACRAWAX C® brand of ethylenbisstearamide and has adiscrete melting point of about 142° C.

In one embodiment of the method, the binder composition comprises theguanidine wetting agent in the range from about 5 wt % to about 30 wt %based on the binder composition, the polycarbonate polymer in the rangefrom about 30 wt % to about 85 wt % based on the binder composition, andthe ethylenebisamide wax in the range from about 10 wt % to about 40 wt% based on the binder composition. In one embodiment of the method, thebinder composition comprises the guanidine wetting agent at about 15.5wt %, the polycarbonate polymer at about 59.4 wt %, andethylenebisstearamide at about 25.1 wt %, each weight percent based onthe binder composition. In one embodiment of the method, thepolycarbonate polymer is Q-PAC™ 40 brand of poly(propylene carbonate),and is present at about 60 wt %. In one embodiment of the method, theethylenebisamide is AGRAWAX® brand of ethylenebisstearamide, and ispresent at about 25 wt %.

In one embodiment of the method, the binder composition comprises theguanidine wetting agent in the range from about 10 wt % to about 25 wt %based on the binder composition, the polycarbonate polymer in the rangefrom about 40 wt % to about 60 wt % based on the binder composition, andthe ethylenebisamide wax in the range from about 15 wt % to about 35 wt% based on the binder composition.

In one embodiment of the method, the binder composition is present in anamount in the range from about 30 vol % to about 60 vol % of the greencomposition and the inorganic powder is present in an amount from about70 vol % to about 40 vol % of the green composition. In one embodimentof the method, the binder composition is present in an amount in therange from about 40 vol % to about 50 vol % of the green composition andthe inorganic powder is present in an amount from about 60 vol % toabout 40 vol % of the green composition. In one embodiment, the greencomposition includes the binder composition in an amount of about 35 vol% and the inorganic powder in an amount of about 65 vol %.

PREPARATION

FIG. 1 is a schematic diagram of the steps in a method of making a partby powder injection molding in accordance with the present invention.FIG. 1 shows a generalized process for powder injection molding whichmay be performed in accordance with the present invention. In a firststep 10 an inorganic powder and a binder composition according to thepresent invention are obtained and combined. In one embodiment, the stepof preparing the binder composition includes steps of mixing, blendingand dispersing the components of the binder composition as needed toprepare a homogenous, or nearly homogenous mixture of the components inthe binder composition, in a powder form. In one embodiment, the bindercomposition and the inorganic powder are first dry blended to produce ahomogenous mix of dry materials. In one embodiment, the bindercomposition is micronized to a size similar to that of the inorganicpowder with which it will be combined to form the green composition. Inone embodiment, the binder composition is ground to a particle size inthe range from about 10 μm to about 100 μm.

In an optional second step (not shown) the inorganic powder and thebinder composition are combined in a premixing of the green composition.The optional premixing step may include mixing in, e.g., a ball mill. Inthis optional step, additional components, if used, may be added andblended into the mixture as desired.

In a step 20 the components of the green composition are fed into a twinscrew compounding extruder. In the step 20, while passing through thetwin screw compounding extruder, the components of the green compositionare subjected to a high shear for effectively combining the inorganicpowder and binder composition. In one embodiment, the output from thetwin screw compounding extruder is a string of the green composition,which is then fed to a pelletizer. In one embodiment, the output fromthe twin screw compounding extruder is pelletized by a pelletizingapparatus directly attached to the extruder apparatus. Forming the greencomposition into pellets facilitates handling, both for immediate andfor subsequent use. The mixing in the twin screw compounding extruder inthe step 20 facilitates blending the various green compositions as maybe required for particular applications. The mixing in the twin screwcompounding extruder in the step 20 combines, compounds and pelletizesthe green composition. The pellets formed by the step 20 are cooled, andmay be stored for later use.

In one embodiment of the step 20 the binder composition is dry blendedwith the inorganic powder prior to feeding to the twin screw compoundingextruder, and the blended components of the green composition are fedinto the extruder together. In one embodiment, the binder compositionand inorganic powder components of the green composition are fedseparately into the twin screw compounding extruder. In one embodiment,the binder composition is fed into the twin screw compounding extruderat a first point, and the inorganic powder component is fed in at asecond point, downstream from the first point. In one embodiment, thetwin screw compounding extruder is a Leistritz 18 mm co-rotating twinscrew compounding extruder. In one embodiment, the Leistritz twin screwextruder has the design shown in FIG. 5. A further description of FIG. 5is provided below. In one embodiment, the green composition exiting thetwin screw compounding extruder emerges in the form of a string, passingonto a conveyor, which is subsequently cooled and then cut into pellets.

Referring still to FIG. 1, in an injection molding step 30, the pelletsof the green composition are heated, melted, mixed and injected into amold having the desired shape of the part of interest. The part formedat this stage is known as a green part or a compact for a part. In oneembodiment, the molten green composition is injected into the mold at apressure in the range from about 100 psi (about 70,307 Kg/m²) to about2000 psi (about 1,406,139 Kg/m²). In one embodiment, the molten greencomposition is injected into the mold at a pressure of about 800 psi(about 562,455 Kg/m²). In the injection step 30, pellets havingdifferent green compositions may be blended. Following the injectionstep 30, the green part is cooled and released from the mold.

In one embodiment, the pellets are fed into a hopper and thence into ahorizontal injection molding machine. In one embodiment, the Injectionmolding machine is a standard injection molding machine used forinjection molding parts in known processes.

In one embodiment, the green part has a green strength in the range ofabout 800 psi (about 562,456 Kg/m²) to about 12,000 psi (about 8,436,835Kg/m²). In one embodiment, the green part has a green strength in therange of about 2000 psi (about 1,406,139 Kg/m²) to about 8000 psi (about5,624,556 Kg/m²). In one embodiment, the green part has a green strengthin the range of about 4000 psi (about 2,812,278 Kg/m²) to about 6000 psi(about 4,218,418 Kg/m²).

The green part is then transferred to a debinding/sintering oven, inwhich one or more steps of debinding 40 are carried out. In oneembodiment, the debinding step 40 includes a plurality of temperatureincreases to elevated temperatures. In one embodiment of the debindingstep 40, each of the elevated temperatures are maintained constant for aperiod of time. In one embodiment of the debinding step 40, the elevatedtemperatures correspond to temperatures at which individual ingredientsof the binder composition are debound. In one embodiment of thedebinding step 40, a first elevated temperature corresponds to thedebinding temperature of the polycarbonate polymer, a second elevatedtemperature corresponds to the debinding temperature of theethylenebisamide wax, and a third elevated temperature corresponds tothe debinding temperature of the guanidine wetting agent. In oneembodiment of the debinding step 40, the third elevated temperature ishigher than the second elevated temperature, and the second elevatedtemperature is higher than the first elevated temperature.

Following the debinding step 40, the green part is subjected to a step50 of sintering. The sintering step 50 may be performed in the same ovenin which the debinding step 40 was performed, or the green part may bemoved to a separate sintering oven for the sintering step 50.

The variables for the debinding process conditions include selection ofthe identity, pressure and flow rate of the atmosphere in the debindingoven chamber, selection of the temperatures for each debinding step,selection of the rate of increase in temperature during the transitionfrom one debinding step to the next, and selection of the time eachdebinding temperature is held while a particular component is deboundfrom the green composition. Additional variables arise from the exactnature of both the components of the binder composition and theinorganic powder used in the green composition. The time period at whicha particular debinding temperature is held during a debinding process isknown as “soaking” the green composition at that temperature. The timeperiods for soaking, and the rate of increase between those temperaturesmust be selected for a given binder composition and a given greencomposition. A certain amount of trial and error is required to optimizethe debinding conditions for a given binder composition and greencomposition. The following general principles may be applied to make aninitial selection of debinding conditions, but the number of variablesmake it likely that some testing will be required.

In selecting the environment for the debinding and sintering, thetemperatures selected for each step of the debinding are primarilyinfluenced by the melting and decomposition temperature of eachcomponent of the binder composition and by the atmosphere in thedebinding oven chamber. However, other factors may be involved as well.

Generally, the temperature at which a part is soaked for removal of eachcomponent during the debinding corresponds to the onset temperature ofits decomposition. In a debinding process, it is helpful if a componentmelts before decomposing, but the important step is the decomposition.If the component melts prior to decomposing, as has been describedherein for the poly(propylene carbonate) polymer, it is helpful to theoverall debinding process due to the relatively small expansion ofvolume in melting as compared to decomposing into gaseous products.Thus, for example, a component may have a certain melting point, such asethylenebisstearamide has a melting point of 142° C., but its debindingvia decomposition is carried out at temperatures in the range from about190° C. to about 225° C., depending on the atmosphere in the debindingoven chamber.

The atmosphere in the debinding over chamber determines the speed ofdebinding at a given temperature. Generally, at a given temperature, anatmosphere of hydrogen results in faster debinding than a vacuum (e.g.,4-12 hours for hydrogen vs. 6-18 hours for vacuum), and a vacuum resultsin faster debinding than an inert atmosphere, for example of argon ornitrogen (e.g., 6-18 hours for vacuum vs. 8-24 hours for an inert gasatmosphere). Alternatively, for a given time for a debinding step, usingan atmosphere of hydrogen allows the debinding step to be carried out ata lower temperature than the same debinding step carried out in avacuum, and a vacuum allows the same debinding step to be carried out ata lower temperature than it would in an inert gas atmosphere. Thus, forexample, a polycarbonate debinding step which may be carried out bysoaking for 60 minutes at 160° C. in a hydrogen atmosphere, would needto be carried out by soaking for 60 minutes at about 190° C. in anitrogen atmosphere. Alternatively, a polycarbonate debinding step whichmay require soaking for 60 minutes at 160° C. in a hydrogen atmosphere,may require soaking for about 90 minutes at 160° C. in a nitrogenatmosphere. The examples provided below provide an indication of thetemperatures and times which may be required for debinding the bindercompositions of the present invention. Suitable atmospheres include,e.g., air, nitrogen, hydrogen, oxygen, argon, and other inert gases.

The pressure and flow rate of the gases used in the debinding ovenchamber provide another variable which must be considered in designing adebinding profile. In a hydrogen atmosphere, the pressure is typicallyfrom about 10% to about 20% above atmospheric, and the hydrogen Ispassed through a 2³ ft chamber at the rate of about 10 ft³/hr (CFH) toabout 15 CFH, or in one embodiment in the same chamber at the rate ofabout 12 CFH. When at atmosphere other than air is used, it is normallyprovided at a super-atmospheric pressure in order to avoid leakageingress of air into the debinding oven chamber. In one embodiment, thepressure in the debinding oven chamber is about 780 torr.Sub-atmospheric pressures may also be used. In one embodiment, a vacuumis placed upon the oven chamber, by reducing the pressure to about 76torr. In other embodiments, similar reduced pressures may be used.Suitable pressures range from a vacuum, i.e. about 10⁻⁵ to about 10⁻⁷torr, to at least about 2 atmospheres, i.e., about 1540 torr. Suitableflow rates range from a flow rate sufficient to produce from about 1atmospheric exchange per hour to a flow rate sufficient to produce atleast about 20 atmospheric exchanges per hour, determined by the volumeof the chamber and the flow rate of gas.

Further variables of properties of the inorganic powder which affecttime and temperature for the debinding steps for a particular greencomposition are: particle size, particle morphology, percent porosityand continuity of porosity. The effects of these variable are complex,and some testing may be required to obtain the optimum for each of theseproperties for a given inorganic powder and binder compositioncombination used in a green composition. For example, decreased particlesize increases the surface area which in turn increases thesinterability to produce fully dense parts. When particles are moreclosely packed, less porosity is formed and the likelihood of porecontinuity decreases. This means the binder composition will be retardedin finding a means of escape from the part as the debinding processproceeds. Thus, the result of smaller inorganic powder particle size islikely to be a longer debind time, since the temperature increases maybe required to proceed at a reduced rate of increase.

A further variable which affects time and temperature for the debindingsteps for a particular green composition is the chemical nature of theinorganic powder. A powder may tend to act as an activator, or even likea catalyst, in the decomposition of one or more of the components of abinder composition, and so may result in faster debinding of thosecomponents. Alternatively, if the inorganic powder is a relatively inertmaterial, such as alumina, Al₂O₃, the primary factors affecting thedebinding process are the temperature, time and atmosphere of thedebinding.

Alternatives to the preparation of green parts as described above by PIMinclude pressing the green composition into a mold for P&S, followed bya sintering step. Alternatively, the blended green composition can beextrusion- or ejection-molded to form a green body, or the green bodycan be prepared by casting the mixture on a tape. The green body mayalso be prepared by spray-drying rotary evaporation, etc. Following theformation of the blended green composition into the desired shape, theshaped mass is subjected to the above described elevated temperaturetreatments. These treatments first eliminate the binder composition, asdescribed more fully above, and then sinter the inorganic powdersresulting in the formation of a shape having the desired propertiesincluding suitable densities.

For metal powders, the sintering generally occurs between about 400° C.to about 2100° C., typically to about 1000° C. For ceramic processes,the sintering generally occurs from about 600° C., preferably about 700°C. up to about 1700° C. Of course, the sintering temperature ischaracteristic of the particular inorganic powder used in the greencomposition, and may be affected by impurities or additives. Forexample, carbonyl iron is frequently doped with nickel, at the level of,for example, about 2 wt %, as a sintering aid. The presence of thenickel allows the sintering to take place at a lower temperature and/orin a shorter amount of time than would otherwise be required forcarbonyl iron. When the inorganic powders are oxide powders, baking andsintering can be effected in the presence of oxygen. When the inorganicpowders are non-oxide powders such as the nitrides and carbides,sintering is effected in a nonoxidizing atmosphere such as an atmosphereof hydrogen, argon or nitrogen gas.

The debinding step takes place at moderately elevated temperatures, andis generally completed by ramping to a series of temperatures belowabout 700° C. It is the debinding steps which are the primary focus ofthe present invention.

Removal of the organic materials of the binder composition is generallycompleted before the inorganic powders are subjected to sintering. Inthis process, substantially all of the binder composition is removed.Some of the binder composition materials may remain following thedebinding, although the amount is relatively small. These remainingportions of the binder composition will be essentially completelyremoved in the sintering steps, depending of course, on factors such asthe decomposition temperature of the remaining binder component, thesintering temperature and the sintering atmosphere.

Each of the three ingredients of the binder composition, thepolycarbonate polymer, the ethylenebisamide and the guanidine wettingagent, may be initially formed in a solid, pelletized form. To form thepellets, these ingredients are combined and heated to melting, atapproximately 100° C., in the manner indicated above. The threeingredients are-partially miscible with each other, so that whenactively mixed in a twin screw compounding extruder at approximately100° C., the binder composition is almost homogenous, and the bindercomposition quickly and easily forms a uniform heterogeneous mixturewith a minimum of shear. Thus, the binder composition forms a uniformheterogeneous mixture with only one extrusion cycle. In one specificcase, the liquid binder composition was mixed at a temperature ofapproximately 100° C. to form a uniform heterogeneous mixture within 10minutes of extrusion.

The heated heterogeneous liquid mixture of the binder composition may bemixed with the inorganic powder to form the green composition. Themixing of the binder composition and inorganic powder to form the greencomposition is best undertaken in the twin screw compounding extruder,which, among other benefits, results in thorough mixing with a minimumof exposure of the green composition components to atmospheric air. Suchexposure may be deleterious to either or both the binder composition andthe inorganic powder.

The green composition, when mixed at a temperature of about 100° C. forma liquid with a viscosity of between 5 and 300 Pascal-seconds dependingon the shear rate. As the shear rate increases, the viscosity generallydecreases to some degree, although as would be understood, there is alimit to the decrease.

The heated green composition may be extruded at approximately 100° C. toform feedstock pellets. The feedstock pellets, once made, may beinjection molded at any subsequent time by heating to a temperature ofapproximately 100° C. and pumping into a mold to make a green part,which is also known as a compact of a part. The resulting green part wasthen subjected to the series of temperature increases to debind thecompact and thence to sinter the inorganic powder, as has been describedabove.

In one embodiment, the method includes, in step (d), a plurality oftemperature increases to elevated temperatures. In one embodiment, themethod includes maintaining each of the elevated temperatures constantfor a period of time. In one embodiment of the method, the elevatedtemperatures correspond to temperatures at which individual ingredientsof the binder composition are debound. In one embodiment, the elevatedtemperatures include a first elevated temperature which corresponds tothe debinding temperature of the polycarbonate polymer, a secondelevated temperature which corresponds to the debinding temperature ofthe ethylenebisamide wax, and a third elevated temperature whichcorresponds to the debinding temperature of the guanidine wetting agent.In one embodiment, the third elevated temperature is higher than thesecond elevated temperature, and the second elevated temperature ishigher than the first elevated temperature.

Debinding of the compact may be completed when the temperature of thecompact reaches about 600° C. The temperature should be maintained atthis level for a period of up to about 12 hours. This heating processremoved the binder composition from the compact. The compact was thensintered by heating the compact to a temperature of approximately 1,650°C. for a period of up to 4 hours. The resulting product is a part madeof the inorganic material of which the inorganic powder had been made.

FIG. 5 is a schematic engineering drawing of one screw 60 of a twinscrew compounding extruder 62 in accordance with one embodiment of theinvention. The twin screw compounding extruder 62 shown in FIG. 5 is aschematic depiction of a Leistritz 18 mm twin screw compoundingextruder, which is used in one embodiment of the method of the presentinvention. The Leistritz twin screw compounding extruder 62 provides ahigh level of combining and compounding the components of the greencomposition of the present invention. As shown in FIG. 5, the screw 60is used in the twin screw extruder 62. The twin screw extruder 62includes a main feed 64, a secondary feed 66 and a vent 68. In oneembodiment, the binder composition is fed into the main feed 64 and theinorganic powder is fed into the secondary feed 66. The vent 68 isprovided to vent entrapped gases and to maintain the internal pressurein the twin screw compounding extruder 62 at a desired level.

EXAMPLES

The following exemplary formulations are intended to provide a betterunderstanding of the invention, and are not intended as limiting.

Example 1

A green composition comprising a binder composition and carbonyl iron,according to the present invention, was prepared as follows.

The binder composition was as follows:

poly(propylene carbonate) Q-PAC ™ 40 59.43 wt % ethylenebisstearamideACRAWAX ® C 25.15 wt % guanidine ethyl hexanoate 8.49 wt % guanidinestearate 6.94 wt % Total 100.0

The binder composition was prepared by combining the ingredients in atwin screw compounding extruder, heating to about 100° C. for about 10minutes, until the mixture is substantially homogenous, and thenpelletizing the binder composition in, e.g., a strand cutter pelletizingapparatus. This binder composition is designated APEX™ 201.

The ingredients for the green composition, comprising 59 vol % carbonyliron doped with 2 wt % nickel powder as a sintering aid, and 41 vol % ofpellets of the above binder composition were combined, compounded andpelletized in a twin screw compounding extruder as described above.Expressed on a weight basis, the green composition comprised 91 wt %carbonyl iron/Ni and 9 wt % of the above binder composition. After thegreen composition was thoroughly compounded, it was extruded andpelletized. The pellets were subsequently fed into an injection moldingmachine, and injected into a mold.

Example 2

A green composition comprising a binder composition and titanium CPpowder, according to the present invention, was prepared as follows.

The binder composition was the same as in Example 1.

The ingredients for the green composition, comprising 59 vol % titaniumCP grade powder, and 41 vol % of the binder composition prepared inExample 1, were combined were combined, compounded and pelletized in atwin screw compounding extruder as described above. Expressed on aweight basis, the green composition comprised 83 wt % titanium OP gradepowder and 17 wt % of the above binder composition. After the greencomposition was thoroughly compounded, it was extruded and pelletized.The pellets were subsequently fed into an injection molding machine, andinjected into a mold.

Example 3

A green composition comprising a binder composition and sub-micronzirconia powder stabilized with yttria, according to the presentinvention, was prepared as follows.

The binder composition was the same as in Example 1.

The ingredients for the green composition, comprising 47 vol % zirconiapowder stabilized with yttria powder, and 53 vol % of pellets of thebinder composition prepared in Example 1, were combined, compounded andpelletized in a twin screw compounding extruder as described above.Expressed on a weight basis, the green composition comprised 80 wt %zirconia/Y₂O₃ powder and 20 wt % of the above binder composition. Afterthe green composition was thoroughly compounded, it was extruded andpelletized. The pellets were subsequently fed into an injection moldingmachine, and injected into a mold.

FIG. 2 is a graph of a debinding profile of a first exemplary greencomposition according to the present invention. The debinding profileshown in FIG. 2 reflects the following steps of a debinding process:

Elapsed Time, Time, Step No. Action in Step min. min. 21 Heat from RT @75° C./hr to 110° C. 68  68 22 Soak (hold) @ 110° C. 60 128 23 Heat from110° C. @ 100° C./hr to 140° C. 18 146 24 Heat from 140° C. @ 75° C./hrto 190° C. 40 186 25 Soak (hold) @ 190° C. 60 246 26 Heat from 190° C. @150° C./hr to 425° C. 94 340 27 Soak (hold) @ 425° C. 60 400 28 Heatfrom 425° C. to sintering temperature

The following binder composition was used in the green composition whichwas subjected to the debinding process shown in FIG. 2:

poly(propylene carbonate) Q-PAC ™ 40 59.43 wt % ethylenebisstearamideACRAWAX ® C 25.15 wt % guanidine ethyl hexanoate 8.49 wt % guanidinestearate 6.94 wt % Total 100.0

In FIG. 2, the poly(propylene carbonate) was debound in steps 21 and 22.The ethylenebisstearamide was debound in steps 23, 24 and 25. Theguanidine wetting agent was debound in steps 26 and 27. Followingsubstantially complete debinding, and the end of step 27, at an elapseddebinding time of 400 minutes, the part was sintered by heating in step28 at the rate of 300° C./hr to a sintering temperature of 1425° C. Inthe steps 21 to 26, the atmosphere was hydrogen at a pressure of 780torr. In the steps 27 and 28, the chamber was held under a vacuum ofabout 10⁻⁶ torr.

FIG. 3 is a graph of a debinding profile of a second exemplary greencomposition according to the present invention. The debinding profileshown in FIG. 3 reflects the following steps of a debinding process:

Elapsed Time, Time, Step No. Action in Step min. min. 31 Heat from RT @75° C./hr to 160° C. 108 108 32 Heat from 160° C. @ 30° C./hr to 210° C.100 208 33 Soak (hold) @ 210° C.  60 268 34 Heat from 210° C. @ 60°C./hr to 325° C. 115 383 35 Heat from 325° C. @ 30° C./hr to 450° C. 250633 36 Soak (hold) @ 450° C.  60 693 37 Heat from 450° C. to sinteringtemperature

The following binder composition was used in the green composition whichwas subjected to the debinding process shown in FIG. 3:

poly(propylene carbonate) Q-PAC ™ 40 59.43 wt % ethylenebisstearamideACRAWAX ® C 25.15 wt % guanidine ethyl hexanoate 8.49 wt % guanidinestearate 6.94 wt % Total 100.0

In FIG. 3, the poly(propylene carbonate) and ethylenebisstearamide weredebound together in steps 31, 32 and 33. The guanidine wetting agent wasdebound in steps 34, 35 and 36. Following substantially completedebinding, and the end of step 36, at an elapsed debinding time of 693minutes, the part was sintered in step 37 by heating at the rate of 300°C./hr to a sintering temperature of 1425° C. The atmosphere was the sameas that set forth above with respect to FIG. 2.

Example 4

A green composition comprising a binder composition and carbonyl irondoped with 2 wt % nickel, according to the present invention, wasprepared as follows.

The binder composition was the same as in Example 1.

The ingredients for the green composition, comprising 51 vol % carbonyliron doped with 2 wt % nickel powder, and 49 vol % of the bindercomposition prepared in Example 1, were combined, compounded andpelletized in a twin screw compounding extruder as described above.Expressed on a weight basis, the green composition comprised 88 wt %carbonyl iron w/ 2 wt % Ni powder and 12 wt % of the above bindercomposition. After the green composition was thoroughly compounded, itwas extruded and pelletized. The pellets were subsequently fed into aninjection molding machine, and injected into a mold.

FIG. 4 is a graph of a debinding profile of the green composition ofExample 4, according to the present invention. Following a five hoursoak at 90° C. to remove surface moisture, which is not shown in FIG. 4,the debinding profile shown in FIG. 4 reflects the following steps of adebinding and sintering process (the sintering steps 52 to 57 are notshown In FIG. 4):

Elapsed Time, Time, Step No. Action in Step min. min. 41 Heat from RT @90° C./hold 5 hours 300  300 42 Heat from 90° C. @ 48° C./hr to 180° C.112  412 43 Soak (hold) @ 180° C.  30  442 44 Heat from 180° C. @ 6°C./hr to 225° C. 450  892 45 Soak (hold) @ 225° C.  30  922 46 Heat from225° C. @ 30° C./hr to 325° C. 200 1122 47 Soak (hold) @ 325° C.  301152 48 Heat from 325° C. @ 15° C./hr to 400° C. 300 1452 49 Soak (hold)@ 400° C.  30 1482 50 Heat from 400° C. @ 78° C./hr to 780° C. 295 177851 Soak (hold) @ 780° C.  60 1838 52 Heat from 780° C. @ 600° C./hr to800° C.  2 1840 53 Soak (hold) @ 800° C.  10 1850 54 Heat from 800° C. @480° C./hr to 1330° C.  68 1918 55 Soak (hold) @ 1330° C.  5 1923 56Heat from 1330° C. @ 60° C./hr to 1380° C.  50 1973 57 Soak (hold) @1380° C.  80 2053

The binder composition shown in Example 1 was used in the greencomposition which was subjected to the debinding process shown in FIG.4. In the debinding process shown in FIG. 4, the atmosphere washydrogen, at a flow rate of 12 CFM and a debinding oven chamber pressureof 780 torr. In the debinding process shown in FIG. 4, thepoly(propylene carbonate, was debound in steps 42 and 43,ethylenebisstearamide was debound in steps 44 and 45. The guanidinewetting agent was debound in steps 46 and 47. Steps 48 and 49 provide anextra backup or “insurance” step to make certain that the debindingprocess is complete. Such “insurance” steps are not always necessary,but may be desirable, particularly during the development of a debindingprotocol. Following substantially complete debinding, and the end ofstep 49, at an elapsed debinding time of 1482 minutes, the part wassintered in steps 50-57, as shown in the foregoing table to a finalsintering temperature of 1380° C. Steps 52-57 are not shown in FIG. 4due to space limitations, however, the profile would continue inaccordance with the data shown in the foregoing table in a mannersimilar to that shown for the debinding steps as shown in the table andFIG. 4. It is noted that the initial step of soaking at 90° C. for fivehours can be eliminated with proper materials handling. If the bindercomposition and the inorganic powder are maintained in a suitably drycondition, a step of drying would not be required.

A wide variety of parts can be made by PIM in accordance with thepresent invention. Such parts include for example, for an inorganicpowder which is a metal, gun parts, shear clipper blades and guides,watch band parts, watch casings, coin feeder slots, router bits, drillbits, disk drive magnets, VCR recording heads, jet engine parts,orthodontic braces and prostheses, dental brackets, orthopedic implants,surgical tools and equipment, camera parts, computer parts, and jewelry.Such parts include for example, for intermetallic inorganic powders,turbochargers, high temperature insulators, spray nozzles and threadguides. Such parts include for example, for ceramic Inorganic powders,optical cable ferrules, ski pole tips, haircutting blades, airfoilcores, piezoelectric (e.g., lead zircon titanate, PZT) parts, oxygensensors and spray nozzles.

BINDER COMPOSITIONS FOR PRESS & SINTER APPLICATIONS

The binder composition of the present invention may also be used forpress & sinter applications. In press & sinter application, theinorganic powder loading is considerably higher than in PIM. Thetrade-off for the higher loading is the limitation that the parts madeby a press & sinter process are quite limited in complexity. In fact,press & sinter can be considered to be limited to quite simple parts.The types of inorganic powders which can be used in press & sinterapplications are more limited, due to the requirement that the powdersbe sufficiently malleable and compactable to be useable in press &sinter applications. Powders having a high hardness values, such as forexample WC, are generally not useable in press & sinter applications.The hardness value becomes an issue in press & sinter applications dueto the low binder loadings used in press & sinter as compared to PIM.

In a press & sinter application, the loading of the binder compositionin the green composition is typically in the range from about 1% byvolume to about 10% by volume of the green composition from which thepart will be formed. (As with PIM applications, the green composition ismeasured on a volume basis, with the loadings expressed in volumepercentages.) In one embodiment, the loading of the binder compositionis 1% by volume. In one embodiment, the loading of the bindercomposition is 2% by volume. In one embodiment, the loading of thebinder composition is 3% by volume. In one embodiment, the loading ofthe binder composition is 4% by volume. In a press & sinter process, thegreen composition is pressed into the desired shape by means of, e.g., ahydraulic press. Once the part is pressed into its shape, it has a greenstrength in the range from about 1,000 psi (about 703,070 Kg/m²) toabout 4,000 psi (about 2,812,278 Kg/m²). The part is then sintered.

For a press & sinter application, the binder composition according tothe present invention has the following ranges of components (aspreviously, the binder composition is prepared on a weight by weightpercentage bases (wt %)).

polycarbonate polymer 10-50 wt % ethylenebisamide wax 30-70 wt %guanidine wetting agent  5-30 wt %

For press & sinter applications, the foregoing descriptions with respectto the selection of polycarbonate polymer, ethylenebisamide wax andguanidine wetting agent continue to apply. Thus, the acid used to formthe reaction product of guanidine and acid is selected on the basis ofthe isoelectric point of the inorganic powder. Similarly, the same rangeof inorganic powders can be used, as long as these are useable in apress & sinter application.

In view of the foregoing description, it is apparent that the presentinvention provides a new and improved binder which is formed and/or usedin accordance with a new and improved method.

What is claimed is:
 1. A method for forming a part by powder injectionmolding including: (a) forming a green composition comprising a bindercomposition and an inorganic powder, wherein the binder compositioncomprises a polycarbonate polymer, an ethylenebisamide wax, and aguanidine wetting agent; (b) heating the green composition to debind thegreen composition by reverse debinding.
 2. The method of claim 1 whereinthe inorganic powder is selected from a metal powder, a metal oxidepowder, an intermetallic powder and a ceramic powder.
 3. The method ofclaim 1, wherein the guanidine wetting agent is a reaction product ofguanidine and an acid selected from organic acid, a fatty acid and analkyl sulfonic acid.
 4. The method of claim 1, wherein the polycarbonatepolymer has a number average molecular weight in the range from about35,000 to about 65,000.
 5. The method of claim 1, wherein theethylenebisamide wax has a discrete melting point in the range fromabout 130° C. to about 150° C.
 6. The method of claim 1, wherein theguanidine wetting agent is present in the range from about 5 wt % toabout 30 wt % of the binder composition, the polycarbonate polymer ispresent in the range from about 30 wt % to about 85 wt % of the bindercomposition, and the ethylenebisamide wax is present in the range fromabout 10 wt % to about 40 wt % of the binder composition.
 7. The methodof claim 1, wherein the binder composition is present in an amount inthe range from about 30 vol % to about 60 vol % of the green compositionand the inorganic powder is present in an amount from about 70 vol % toabout 40 vol % of the green composition.
 8. The method of claim 1,wherein step (b) includes a plurality of temperature increases toelevated temperatures.
 9. The method of claim 8, wherein each of theelevated temperatures are maintained constant for a period of time. 10.The method of claim 8, wherein the elevated temperatures correspond totemperatures at which individual ingredients of the binder compositionare debound.
 11. The method of claim 8, wherein a first elevatedtemperature corresponds to the debinding temperature of thepolycarbonate polymer, a second elevated temperature corresponds to thedebinding temperature of the ethylenebisamide wax, and a third elevatedtemperature corresponds to the debinding temperature of the guanidinewetting agent.
 12. The method of claim 11, wherein the third elevatedtemperature is higher than the second elevated temperature, and thesecond elevated temperature is higher than the first elevatedtemperature.
 13. The method of claim 1, further comprising a step ofinjecting the green composition into a mold for powder injectionmolding.
 14. The method of claim 1, further comprising a step of gravityfeeding the green composition into a mold for press and sinter molding.15. The method of claim 1, further comprising a step of heating the partto a temperature at which the inorganic powder is sintered.
 16. A methodfor forming a part by powder injection molding, comprising: (a) forminga green composition comprising a binder composition and an inorganicpowder, wherein the binder composition comprises a polycarbonatepolymer, an ethylenebisamide wax, and a guanidine wetting agent; (b)transferring the flowable green composition into a mold for a part; (c)heating the part to debind the green composition by reverse debinding;(d) heating the part to a temperature at which the powder is sintered.17. A method for forming a part by powder injection molding, comprising:(a) forming a green composition comprising a binder composition and aninorganic powder, wherein the binder composition comprises apolycarbonate polymer, an ethylenebisamide wax, and a guanidine wettingagent; (b) transferring the flowable green composition into a mold for apart by injecting the green composition into the mold for powderinjection molding; (c) heating the part to a plurality of elevatedtemperatures to debind the green composition by reverse debinding,wherein a first elevated temperature corresponds to the debindingtemperature of the polycarbonate polymer, a second elevated temperaturecorresponds to the debinding temperature of the ethylenebisamide wax,and a third elevated temperature corresponds to the debindingtemperature of the guanidine wetting agent. (d) heating the part to atemperature at which the powder is sintered.